The field of the present invention is air/fuel control systems for internal combustion engines.
Various methods for controlling air/fuel mixtures in fuel injection type internal combustion engines have been proposed. Generally, air/fuel mixtures are controlled by regulation of the opening time of a fuel injection device. The opening time is usually determined by multiplying a reference value, T.sub.i, by various correction coefficients which are determined according to operating conditions. The reference value, T.sub.i, is read out from a basic T.sub.i map according to detected values of absolute pressure in the intake passage of the engine and the engine speed.
However, in an internal combustion engine, especially one with a supercharger, a back pressure is often generated which can affect the amount of air which enters the combustion chamber and thus the air/fuel ratio. When an engine with a supercharger is accelerating or decelerating, the back pressure varies greatly due to the inertia of the supercharger. Therefore, the optimum amount of fuel or air to be supplied cannot be determined with only the intake pipe absolute pressure and the engine speed.
A supercharger is a unit for pre-compressing intake air or air/fuel mixture. The supercharger makes use of the pressure generated by gases in the exhaust passage to turn an exhaust turbine which drives a compressor in the intake passage. During acceleration of the engine, the inertia of the exhaust turbine causes a delay in its increase of rotation, causing the engine back pressure to become higher than it would be during normal operation (that is, when the engine is not accelerating or decelerating). As a result, the amount of air introduced into the combustion chamber is lower during acceleration than it would be during normal operation. Therefore, if the same amount of fuel is supplied as is provided during normal operation, the air/fuel mixture becomes overrich, resulting in an increase of fuel consumption, deterioration of exhaust gas characteristics and reduction of engine output. On the other hand, during deceleration the inertia of the supercharger turbine causes a delay in its decrease of rotation, resulting in a back pressure which is lower than that of normal operation and thus a higher amount of air introduction. Therefore, if the normal amount of fuel is supplied, the air/fuel mixture becomes overlean, causing engine performance to deteriorate.
One example of a method for controlling the air/fuel mixture of an internal combustion engine is found in U.S. Pat. No. 4,509,485.